In the beginning printers aimed to make their work as similar as possible to the hand-written books that had preceded them. As there are today, there were conventional sizes for different types of books: for example, brevier comes from breviary and became the name for a type size (approximating to 8 point). A pica was an ecclesiastical directory giving the rules for fixing the dates of moveable church feasts. It perhaps has no relevance to the history of type, but pica is also the Latin for magpie, and the OED speculates that there may have been some etymological connection between the bird and the book. Obviously directories of moveable feasts must have been of such a size that to fit all the information onto the page you had to write it in script which was about as large as today’s 12pt type.
Like so much in the world of printing and publishing the exact size of a pica has fluctuated with time and location. A pica is 12 points: that much has remained constant. The size of a point is really the variable: in the age of computers it is now defined as 1/72”, so the pica is 1/6”.
Traces of the old names for type sizes: bourgeois (pronounced bur-joice like rejoice), minion, canon, primer (pron. primmer), and others named after precious stones; pearl, agate, ruby, can still be found today in the names publishers give to their different sizes of bibles.
The sizes given in the list below are approximate. When fonts were thus named different printer would cast types to various sizes. Overall the sizes thus named conformed roughly to the point sizes shown.
Diamond 4½ pt
Agate 5½ pt
Ruby 5½ pt
Long Primer 10pt
Small Pica 11pt
Great Primer 18pt